Liquid-liquid extraction, commonly known as solvent extraction, is a technique for separating the components of a solution by distribution between two liquid phases [T C Lo; M H I Baird; C Hanson (Ed), Handbook of Solvent Extraction, John Wiley & Sons, New York, 1983]. With increasing understanding of solvent extraction, it was first shown as a possible technique to separate alcohols from water in the 1940s [D F Othmer; E Trueger, Transact. AIChE, 1941, 37, 597]. The energy crisis in the 1970s forced the scientific community to look for alternatives to conventional petroleum-based fuels and to use alcohols as additives to gasoline. It is only since then that significant attention has been paid to the use of solvent extraction as a possible alternative to distillation for alcohol separation and concentration [J W Roddy, Ind. Eng. Chem. Process Des. Dev., 1981, 20, 104-108].
The choice of a solvent to be used to achieve effective separation of the constituents of a given solution is guided by an estimation of two parameters, namely the equilibrium distribution coefficient (KD) and the separation factor (α). In the context of extraction of alcohol from an aqueous solution, the equilibrium distribution coefficient may be defined as the ratio of the weight fraction of alcohol in the added solvent to that in the aqueous phase. A high value of KD requires only a low solvent-to-water ratio for effective extraction. The separation factor, defined as the ratio of the equilibrium distribution coefficient of alcohol to that of water, gives a measure of the selectivity of the added solvent in extracting alcohol preferentially over water [C L Munson; C Judson King, Ind. Eng. Chem. Process Des. Dev., 1984, 23, 109-115]. The solvent should also have a high capacity for the alcohol as well as low solubility in water. The density difference between solvent and aqueous phases should be such that rapid phase separation takes place [R D Offeman; S K Stephenson; G H Robertson; W J Orts, Ind. Eng. Chem. Res., 2005, 44, 6789-6796]. Along with the estimation of these parameters, two additional considerations need to be kept in mind while evaluating the performance of a solvent. Since alcohol has to be removed from the fermentation broth, care needs to be taken to ensure bio-compatibility of the solvent so that it does not result in the death of the cells carrying out fermentation. Moreover, obtaining a solvent phase rich in alcohol is only one half of the process. Release of alcohol from the solvent into a desired medium and subsequent solvent regeneration form the other half to complete the process. Therefore, the solvent chosen should be such that both of the above mentioned steps can be carried out with technical ease and in an economically viable way.
The ability of different organic solvents to extract ethanol from its dilute solution has been studied extensively in literature. Roddy [J W Roddy, Ind. Eng. Chem. Process Des. Dev., 1981, 20, 104-108] was the first to identify the need for comparative study of performance of different solvents and carried out experiments to evaluate KD and α for a variety of solvents. It was found that the ethanol extracting potential of the solvent increased in the following order: hydrocarbon=halocarbon<ether<ketone<amine<ester<alcohol.
It was suggested that for a solvent to be effective, keeping process design considerations in mind, we need KD,ethanol>0.5 and α>10. Munson and King [C L Munson; C Judson King, Ind. Eng. Chem. Process Des. Dev., 1984, 23, 109-115] also carried out a study similar to that of Roddy and highlighted the trade off between equilibrium distribution for ethanol and selectivity. They also found that solvents which behaved as Lewis acids had a more favorable combination Of KD,ethanol and α than did other solvents. They reported that solvent mixtures could have values of KD,ethanol and α which were vastly different from what would be obtained by linear interpolation of the values for the solvent constituents. The values obtained by Munson and King supported Roddy's [J W Roddy, Ind. Eng. Chem. Process Des. Dev., 1981, 20, 104-108] findings of alcohols, acids and ketones being good solvents for ethanol extraction. In particular, MIBK as well as a 62.8% w/w mixture of 2-ethylhexanoic acid with MIBK were found to be effective in extracting ethanol. Alcohols and esters have also been found to be effective solvents for ethanol extraction by Koullas et al. [D P Koullas; O S Umealu; E G Koukios, Sep. Sci. Technol., 1999, 34, 2153-2163].
Recently, Offeman and co-workers [R D Offeman; S K Stephenson; G H Robertson; W J Orts, Ind. Eng. Chem. Res., 2005, 44, 6789-6796; and R D Offeman; S K Stephenson; G H Robertson; W J Orts, Ind. Eng. Chem. Res., 2005, 44, 6797-6803] focused their attention on the ethanol extraction potential of alcohols and studied the effect of the alcohol molecular weight and structure on this potential. KD,ethanol and KD,water have been found to be dependent on the point of branching of the carbon chain in the alcohol, length of the branches and the main chain and location of the hydroxyl group along the chain length. Alcohols containing rings were found to have values of a less than the aliphatic alcohols. The trade off between KD,ethanol and α was reconfirmed.
Butanol, like ethanol, is produced by fermentation of sugars or starches. Several technologies, apart from distillation, have been tested for separating the butanol produced from fermentation broths. Liquid-liquid extraction has been shown by Groot et al. [W J Groot; H S Soedjak; P B Donck; R G J M van der Lans; K C A M Luyben; J M K Timmer, Bioprocess. Eng., 1990, 5, 203-216; and W J Groot; R G J M van der Lans; K C A M Luyben, Process Biochem., 1992, 27, 61-75] to be one of the techniques with the greatest potential because the solvents can be selected to give a high selectivity for butanol over water and also because of the possibility to carry out the extraction in-situ. Solvents like kerosene, dodecanol, tetradecanol, oleyl alcohol and benzyl benzoate have been tested for the ability to extract ethanol by Roffler and co-workers [S R Roffler; H W Blanch; C R Wilke, Bioprocess. Eng., 1987, 2, 1-12]. Oleyl alcohol and 50 wt % of oleyl alcohol in benzyl benzoate have been shown to be most effective among the solvents tested, for the extraction of butanol.
Solubility parameters are used to assign a numerical value to the solvency behavior of a specific solvent. They have been widely used to aid in solvent selection. A single value of a solubility parameter for each solvent, as proposed by Hildebrand, did not completely capture the several interactions that take place between the solvent and solute molecules and lead to the solvation. To overcome this drawback, Hansen proposed the use of a set of three solubility parameters to describe a solvent's ability to dissolve a solute. The three parameters accounted for the dispersion characteristics (δD), the polar characteristics (δP), and the hydrogen bonding ability (δH) of the solvent molecule. Hansen plotted the location of different solvents in a three dimensional space formed by the three solubility parameters. It is found that in this three dimensional “solubility space”, there exists a region around each solvent, molecules lying in which can be dissolved by the solvent. If the dispersion parameter is doubled, this region assumes a spherical shape centered on the location of the solvent of interest. The radius of this solubility sphere, called the interaction radius (Ro), is characteristic of each solvent molecule and can be determined experimentally. Distance between two molecules in this modified “solubility space”, Ra, can be calculated according to the following formula:(Ra)2=4(δD2−δD1)2+(δP2−δP1)2+(δH2−δH1)2  (1)
Hence, the condition for solubility of one substance in another can be mathematically expressed as:Ra/Ro≦1  (2)
The three solubility parameters for a wide range of solvents and polymers as well as their interaction radius are reported in the literature [C M Hansen, Hansen Solubility Parameters: A User's Handbook, 2nd, CRC Press; 2007]. A rough estimate of Ro for a substance can be made by calculating the distance of its molecule from various solvents, which have been shown in literature to dissolve the substance of interest in the “solubility space”, and choosing the largest among the calculated values.
The change in the volume of a gel in response to change(s) in its environment is known as the volume phase transition of the gel. Since the discovery of this interesting phenomenon in 1978 [T Tanaka, Phys. Rev. Lett., 1978, 40, 820-823], it has been the subject of extensive research. For example, Shibayama and Tanaka in their review of gel properties [M Shibayama; T Tanaka, “Volume Phase Transition and Related Phenomena of Polymer Gels”, In Responsive Gels: Volume Transitions I. Edited by K Dusek: Springer; 1993, 1-62. Advances in Polymer Science, vol 109] describe the thermodynamic behavior of polymer gels using the Flory-Rehner model [P J Flory, Principles of Polymer Chemistry. Ithaca, Cornell University Press; 1953] which describes the free energy change involved in the mixing of solvent with a polymeric gel network as a function of the volume fraction of the polymer, volume of each lattice site, temperature and Flory's interaction parameter, χ. The phase diagram thus obtained predicts discontinuous volume phase transitions in polymer solutions at different temperatures. More advanced theories predicting similar volume phase transition in gels and their equation of state can be found in the work by Khokhlov and Nechaev [A R Khokhlov; K A Khachaturian, Polymer, 1982, 23, 1742-1750]. Amiya and Tanaka have shown that discontinuous volume phase transition is not restricted only to synthetic gels (like acrylamide gels) but is a universal property of all gels [T Amiya; T Tanaka, Macromolecules, 1987, 20, 1162-1164].
Volume phase transition of gels is a consequence of changes in either intra-molecular interactions or the interactions between the polymer gel and the solvent medium. These interactions fall in one of the following categories: van der Waals interactions, hydrophobic interactions, hydrogen bonding and electrostatic interactions [F Ilmain; T Tanaka; E Kokufuta, Nature, 1991, 349, 400-401]. The swelling behavior of acrylamide gels in response to changes in the concentration of solvent or system temperature has been attributed to the van der Waals effect [T Tanaka; D Fillmore; S-T Sun; I Nishio; G Swislow; A Shah, Phys. Rev. Lett., 1980, 45, 1636-1639]. The swelling behavior of natural polymers like DNA when the solvent quality is changed has also been shown to be a result of van der Waals interaction [T Amiya; T Tanaka, Macromolecules, 1987, 20, 1162-1164]. The increase in entropy of the system due to break-down of the ordered structure of water molecules in the vicinity of hydrophobic segments of polymer gels in response to increase in temperature, leads to swelling of the non ionic gels like N-isopropylacrylamide gels [Y Hirokawa; T Tanaka, J. Chem. Phys., 1984, 81, 6379-6380]. Also, it has been found that the temperature at which volume phase transition due to hydrophobic interaction takes place is dependent on the surface area of the hydrophobic segments of the gel [H Inomata; S Goto; S Saito, Macromolecules, 1990, 23, 4887-4888]. Ilmain and co workers [F Ilmain; T Tanaka; E Kokufuta, Nature, 1991, 349, 400-401] have proven that the swelling and collapse of the interpenetrating polymer network formed by poly(acrylamide) and poly(acrylic acid) when the temperature is cyclically varied is due to the hydrogen bonding between the acid and amide groups in the molecule. Addition of urea, which is known to disrupt hydrogen bonding, caused the volume phase transition to disappear and the gels were found to be swollen at all temperatures. If the polymer chain has charged groups attached to the backbone, ionic interactions come into play. Since ionization of the groups depends on the pH of the medium, gels can be swollen or contracted by repulsive and attractive electrostatic interactions respectively as the pH of the medium is varied [A Myoga; S Katayama, Polym. Prep. Japan, 1987, 36, 2852-2854].
As discussed above, the change in interactions within a gel system can be triggered by changing any of a number of different stimuli. The most commonly used stimulus is temperature [F Ilmain; T Tanaka; E Kokufuta, Nature, 1991, 349, 400-401; and T Tanaka; D Fillmore; S-T Sun; I Nishio; G Swislow; A Shah, Phys. Rev. Lett., 1980, 45, 1636-1639]. A change in the solvent quality can also cause the gel swelling or contraction [T Amiya; T Tanaka, Macromolecules, 1987, 20, 1162-1164]. Gels which have charged groups respond to changes in pH of the solvent medium [A Myoga; S Katayama, Polym. Prep. Japan, 1987, 36, 2852-2854]. Gels may be designed to respond to light. Light can either cause ionization of groups within the polymer, thus leading to charge interactions [A Mamada; T Tanaka; D Kungwatchakun; M Irie, Macromolecules, 1990, 23, 1517-1519], or can cause local heating of polymer segments [A Suzuki; T Tanaka, Nature, 1990, 346, 345-347]. Gels may also be designed to respond to presence of some particular molecules [E Kokufuta; T Tanaka, Macromolecules, 1991, 24, 1605-1607] and stress [A Onuki, J. Phys. Soc. Jpn., 1988, 57, 1868-1871; and A Onuki, J. Phys. Soc. Jpn., 1988, 57, 703-706] but examples of systems which respond to these stimuli are fewer in literature.
The discovery of ferrocene in early 1950s [T J Kealy; P L Pauson, Nature, 1951, 168, 1039-1040] drew the attention of the scientific community to this metallocene, as it possessed a vast organic chemistry, enabling easy attachment of the ferrocene group to other organic compounds, and also underwent reversible one electron oxidation to ferrocenium ion [I Manners, “Side-Chain Metal-Containing Polymers”, In Synthetic Metal-Containing Polymers. Edited by I Manners: Wiley-VCH Verlag GmbH & Co. KGaA; 2004, 39]. For the first couple of decades after the discovery, the chemists around the world made efforts to understand the chemistry of ferrocene. This was followed by reports of several possible practical applications of ferrocene and its derivatives. The applications can be broadly grouped in the following classes: components of redox systems, effective nontoxic medicinal substances, absorbers of different types of radiation, and others [A N Nesmeyanov; N S Kochetkova, Russ. Chem. Rev., 1974, 43, 710-715].
Saji and coworkers pioneered the control of a molecule's properties by manipulating the oxidation state of the ferrocenyl group attached to the molecule. The ferrocenyl group attached to the head [T Saji; K Hoshino; S Aoyagui, J. Amer. Chem. Soc., 1985, 107, 6865-6868] or the tail [T Saji; K Hoshino; S Aoyagui, J. Chem. Soc., Chem. Commun., 1985, 865-866] of surfactant molecules was reversibly oxidized and reduced, and the corresponding change in the dynamic properties, like the self-assembling behavior, of the micelles was observed. It was found that the introduction of charge in the surfactant molecules by oxidation of the ferrocenyl group led to disruption of the surfactant's micellar structure. This was attributed to an increase in the hydrophilicity of the molecule. The phenomenon of disruption of micellar structure has been used for formation of organic thin films [T Saji; K Hoshino; Y Ishii; M Goto, J. Amer. Chem. Soc., 1991, 113, 450-456; and K Hoshino; T Saji, J. Amer. Chem. Soc., 1987, 109, 5881-1883]. Abbott and his group used the technique developed by Saji and coworkers [T Saji; K Hoshino; Y Ishii; M Goto, J. Amer. Chem. Soc., 1991, 113, 450-456] for the synthesis of surfactant molecules with attached ferrocenyl group and studied the change in surface tension of the aqueous solution of the surfactant on oxidation of the ferrocenyl group [S S Datwani; V N Truskett; C A Rosslee; N L Abbott; K J Stebe, Langmuir, 2003, 19, 8292-8301; B S Gallardo; M J Hwa; N L Abbott, Langmuir, 1995, 11, 4209-4212; and B S Gallardo; K L Metcalfe; N L Abbott, Langmuir, 1996, 12, 4116-4124].
Applications of polymer gels containing ferrocenyl groups have been reported in the literature. Such gels, referred to as redox gels, have been widely shown to work as electrochemical biosensors. Bu et al. synthesized polyacrylamide gel with attached ferrocenyl group and containing entrapped glucose oxidase [H-z Bu; S R Mikkelsen; A M English, Anal. Chem., 1995, 67, 4071-4076]. The gel was attached to a carbon paste electrode. Current flowing during cyclic voltammetry of the electrode dipped in an electrolyte solution was shown to be sensitive to the presence of glucose in the solution. Recently Tsiafoulis et al. have developed a glucose biosensor based on ferrocene intercalated vanadium pentoxide xerogel/polyvinyl alcohol composite film [C G Tsiafoulis; A B Florou; P N Trikalitis; T Bakas; M I Prodromidis, Electrochem. Commun., 2005, 7, 781-788].
Tatsuma et al. synthesized a temperature responsive redox gel of N-isopropyl-acrylamide (NIPA), vinylferrocene (VF) and N,N′-methylenebisacrylamide (BIS) [T Tatsuma; T Kazutake; H Matsui; N Oyama, Macromolecules, 1994, 27, 6687-6689]. It was shown that the increase in hydrophilicity of the redox gel on oxidation of the ferrocenyl groups caused an increase in the phase transition temperature of the gel. The results of this work demonstrate that properties of gels containing ferrocenyl groups are sensitive to the oxidation state of the group and that hydrophilicity of the gels increases when the ferrocenyl groups are charged.